The invention relates to high pressure mixing heads for at least two reactive components for reaction injection molding or reinforced reaction injection molding processes, whereby such components are mixed and the resulting mixture fed to a mold cavity. More particularly, the invention relates to a high pressure mixing head, having at least one reactive component injection valve of the invention, that mixes a plurality of reactive polymeric components for subsequent injection into a mold.
Reaction injection molding, also called liquid injection molding, is a technique for combining liquid reactive components and injecting them into a mold where they rigidify to form a finished polymeric product. The component combination may be achieved by directing streams of two or more liquid reactive components, each under high pressure, to cause their impingement at a common point in a mixing chamber of a mixing head. The resultant component impact creates a homogeneous mix of material in the mixing chamber, which is then either injected under pressure into a closed mold to which the mixing head is connected, or the mix may simply be dispensed into an open mold. Reinforced reaction injection molding is a variation of that process in which one of the liquid reactive components is mixed with a reinforcing material, such as glass fiber or the like, before being introduced into the mixing chamber.
In the production of urethane products, for example, a diisocyanate or polyisocyanate component is reacted with a diol or a polyol component to produce the reaction mixture by separately feeding these components into a mixing chamber, effecting impingement mixing, and thereafter displacing the intimately-formed mix from the chamber into a mold in which the mix can set. It is also known to include in one or both of the components, or their mixture, an additional foaming or blowing agent capable of expanding the polymerizing resin to form cells or pores therein. Expanding agents suitable for this purpose include those which are normally liquid but volitize at the mold temperature, those which are gaseous and are held under pressure until the material is introduced into the mold, and those which are released by chemical action during the mixing stage and thereafter.
In general, the conditions under which the two components are mixed require that the two components be held separate from one another until the instant at which they enter the mixing chamber, since any premature contact of the two components with one another will result in hardening of the materials. Such premature contact often results in the formation of a mass obstructing further outflow of one or both of the components or the mixture. In order to prevent such obstructions, both components are generally provided in a highly flowable form and are circulated by pumps or the like, being provided to the mixing chamber by various means only when reaction to produce the product mix for molding is desired.
Various structures have been proposed for mixing head devices for mixing the reaction components and feeding the resulting mixture to mold means. Such structures include those illustrated in Keuerleber et al., U.S. Pat No. 3,706,515; Wingard et al., U.S. Pat. No 4,082,512; Wingard, U.S. Pat. No. 4,108,606; Leidal, U.S. Pat. No. 4,099,919; Schneider, U.S. Pat. No. 4,239,732; Fiorentini, U.S. Pat. No. 4,332,335; Boden et al., U.S. Pat. No. 4,378,335; Proksa et al., U.S. Pat. No. 4,389,375; Schneider, U.S. Pat. No. 4,440,500; Proksa et al., U.S. Pat. No. 4,442,070; Proksa et al., U.S. Pat. No. 4,452,917; Schneider U.S. Pat. No. 4,452,919; Schmitz et al., U.S. Pat. No. 4,464,056; Schmitz et al., U.S. Pat. No. 4,497,579; Muller et al., U.S. Pat. No. 4,474,310; and Muhle, U.S. Pat. No. 4,115,299. Mixing head structures also include those illustrated in various manufacturer's publications, such as the Krauss Maffei Journal, No. 2/1982, "Polyurethane RIM Technology," pp. 3-4; Hennecke, Brochure Ti 33, "PUR Reaction Casting Machines, type HK," pp. 9-13; and Battenfeld, "Machinery and Equipment for Processing Polyurethane," pp. 12-13.
One form, illustrated in Keuerleber et al., U.S. Pat. No. 3,706,515, includes a body that has an elongated bore which defines a mixing chamber. A plurality of nozzle orifices open into the mixing chamber for conducting reactive polymeric components thereto. The orifices are ordinarily directed at a common point in the mixing chamber to effect impingement of each component with all others to accordingly mix the components together into a homogeneous fluid mass. Flow of all components through all nozzle orifices is simultaneously controlled by a plunger mounted for axially reciprocal movement in the mixing chamber. When the plunger is retracted into its injection position, the orifices in the mixing chamber are opened, permitting reactive components to issue therefrom in the form of high velocity impinging streams. The mixing head also includes a plurality of return passage means, each of which opens into the mixing chamber at a location axially displaced from its respective component nozzle orifice. The plunger is formed with a plurality of axially extending by-pass channels which respectively communicate, when the plunger is in an extended, recirculation position, between the nozzle orifice and return passage means for each component, whereby a closed loop leading back to the component supply is established, providing recirculation for all components simultaneously. Recirculation occurs only via a path external to the nozzle orifices and related valve means. It is not possible to recirculate components individually in the Keuerleber et al. mixing head, nor is it possible to set the recirculation pressure of each recirculating component by means located at the head, individually or otherwise.
Another form of mixing head involves the provision of a second chamber leading to the mold and at substantially right angles to the mixing chamber, this second chamber being provided in turn with a second piston, plunger or ram to drive the mixture out of the chamber. Fiorentini U.S. Pat. No. 4,332,335 discloses a high pressure mixing head in which the additional chamber has the effect of quieting the highly turbulent mixture driven from the mixing chamber into that chamber. The second piston in the quieting chamber serves to clear the channel at the end of each mixing phase and thus prevent the channel from being plugged up by the reacting mixture. The structure of the first chamber and plunger, and the recirculation means of that plunger (axially extending by-pass channels), are substantially identical to that illustrated by Keuerleber et al. Again, recirculation occurs only via a path external to the reactive component injection valves and related apparatus. It is not possible to recirculate components individually. nor is it possible to set the recirculation pressure of each recirculating component by means located at the mixing head, individually or otherwise.
Another form of mixing head involves the provision of means in the reactive component injection valves which allow for recirculation of reactive components internally through the valve when mixing of that component is not ongoing. Schneider U.S. Pat. No. 4,239,732 illustrates a complex reaction injection component valve mechanism utilizing the combination of a solid metering plunger and a reciprocating valve member having an internal passageway with a constricted outlet through which the reactive component must flow. The force of the fluid provided by the acceleration resulting from passage through the restriction impacts on the solid piston when it is in an extended, recirculation position, which stops supply of the component into the mixing chamber and initiates component recirculation. Schneider provides no mechanism in the injection valve for stopping and starting the flow of reactant into the mixing chamber--the injection port is simply blocked by the solid piston (see FIGS. 3-4). No mechanism is provided for selecting the pour pressure or varying the flow into the mixing chamber from said valve. The Schneider valve will not function if the plunger is provided with any type of recirculation or bypass channel, such as disclosed by Keuerleber et al. U.S. Pat. No. 3,706,515, to provide a recirculation path external to the injection valve.
Yet another form of mixing head illustrated in Boden et al. U.S. Pat. No. 4,378,335 involves the provision of external recirculation through axially extending by-pass channels in a metering piston with an injection valve which, although incapable of internal recirculation, does provide means for selecting the pour pressure or varying the flow of reactive component into the reaction chamber (FIG. 1). The valve comprises only one reciprocating member, which opens and closes the valve and controls entry of reactive component to the mixing changer. Not only does Boden et al. fail to provide for internal recirculation in said valve, but it also fails to provide any mechanism in said valve for varying the flow and/or back pressure of the recirculating reactive component.
Boden et al. also discloses a second form of mixing head (FIG. 2), which does not provide for recirculation external to the injection valve through the metering piston, incorporating instead a solid piston. Means are provided for selecting at least two pour pressure/flow varying positions of the single reciprocating member, whereby the opening and closing of the valve and entry of reactive component to the mixing chamber is controlled. While allegedly providing for internal recirculation of reactive component through the valve, the actual structure of the valve in FIG. 2 is plainly inoperative for that function. No provision in that valve is made for any mechanism to vary the flow and/or back pressure of the recirculating reactive component.
Each of these mixing heads suffered from a variety of serious shortcomings and problems. Valves affording external recirculation only through axially-extending metering plunger by-pass channels require that all reactive components supplied to the mixing chamber recirculate simultaneously. With those designs, it is impossible to selectively recirculate some, but not all, of the reactive components provided to the mixing chamber. Valves affording only on/off flow of a reactive component to the mixing chamber require the time consuming and expensive replacement of nozzle means or nozzle orifices to effect a change in pour pressure or amount of material provided to the mixing chamber. Valves affording internal recirculation, but neither external recirculation through metering plunger by-pass channels nor adjustment of recirculation back pressure in said valve, make difficult the appropriate setting of recirculation back pressure, and maintenance of the recirculation pressure close to the pour pressure at a point close enough to the mixing chamber to enable virtually instantaneous changeover of reactive component from recirculation to injection modes, as well as losing the ability to provide rapid cycling through external, axially-extending metering plunger by-pass channel recirculation for all components simultaneously where the same reactive components are continuously being provided to said mixing chamber.
With the advent of multiple, particularly dual density or dual firmness urethane products, the shortcomings of the available mixing heads became acute. The preparation of dual firmness articles requires high pressure mixing head apparatus having the capability to mix reactive components interchangeably, thereby forming two different polymeric densities in the molded product.
One mechanism for production of dual density articles involved the use of two separate high pressure mixing heads, one capable of mixing reactive chemicals resulting in polymeric material of a first density, the other capable of mixing different reactive chemicals resulting in material of a second density. Each head had to discharge the formulation into the same mold for production of the final product. The use of two heads had many disadvantages, including substantial expense, the weight of two heads (which makes the use of robotic apparatus to provide formulation to the molds impossible in most instances, because of the limited weight-bearing capacities of available apparatus), the complexities of supply and recirculate hosing and attendant equipment which the use of two heads entailed, and the complexity and expense of system controls which were necessary to coordinate the operation of the two heads.
The manufacture of dual density products requires the ability to change the chemical composition of the formulation almost instantaneously, particularly where robotic apparatus is used to transport the mixing equipment and manipulate that equipment to lay down different formulations in a pattern in the mold. The need to make the changeover in the formulation "on the fly" presented additional problems over those already present in available mixing head apparatus, particularly if a single high pressure mixing head was to supply dual density material.
The rapid change in density of material which would have to be satisfied in one head would require the combination of one reactive species, A, with another, B, at a first time (for example, the impingement mixing of a polyol 1 with isocyanate), while a third reactive species, A*, was recirculating in the system in some manner. The recirculating material would have to be maintained at a recirculation pressure very close to the desired pour pressure for that material, because there would be no time to build up pressure when formulation change was demanded. When the second density formulation was called for, the flow of first reactive species, A, into a mixing area would have to stop instantaneously, that material recirculated in some manner, and the A* reactant switched from recirculation to introduction to that same mixing chamber for impingement mixing with reactant B. At the same time, the formulation would have to be delivered from the head to the mold, and the mixing chamber and any quieting chamber kept clear of formulation to prevent fouling. Because of the combination of rapid formulation composition change and the need to expel formulation from the head while preventing fouling, the demands upon the recirculation capabilities of such a system is beyond that previously provided for.
There is thus a need for mixing head apparatus that would provide dual density reactive component formulation for introduction into molds, useful for reaction injection molding and/or reinforced reaction injection molding processes, that
1. Requires only one mixing head to provide dual density formulations for the production of molded polymeric products;
2. Provides a choice of internal and external (with respect to the reactive component injection means) recirculation paths through the mixing head for each reactive component, to allow rapid changeover between different density formulations, yet provide flexibility and appropriate transport and clearance of mixed reactants from the head;
3. Provides an internal recirculation path independent of an external path using axially extending by-pass channels in a metering plunger for each reactive component, thereby allowing independent injection and recirculation capabilities for each reactive component affected by density changeover, yet able to make virtually instantaneous changeover at the desired pour pressure;
4. Provides for "in the head", independent adjustment and setting of both pour and recirculation pressures for each reactive component, whereby the necessary pressure balances for rapid changeover could be established at a point as close as possible to the impingement mixing location in the mixing chamber, while providing for more rapid system set-up and independent alteration of previously set pressures with convenience and speed; and
5. Operates at high pressure, assuring excellent mixing through impingement techniques.
No available mixing head has combination of these features, particularly failing to provide the choice of recirculation paths, including an internal recirculation path independent of an external path via axially extending bypass channels in a metering plunger, and "in the head" adjustment of both pour and recirculation pressure.